1. Field of the Invention
The present invention relates to a stencil printing machine in which printing pressure is made variable to adjust the density during printing.
The present application is based on Japanese Patent Application No. 2000-056116, which is incorporated herein by reference.
2. Description of the Related Art
As shown in FIG. 5, a stencil printing machine generally has a porous rotary cylindrical drum 114 to an inner periphery of which ink is supplied and which is disposed in such a manner as to be rotatable about its own axis. A stencil sheet M subjected to a stencil making process is wrapped around an outer peripheral surface of the rotary cylindrical drum 114. Further, a press roller 118 is disposed on the lower side of the rotary cylindrical drum 114. As a printing sheet P is supplied in synchronism with the rotation of the rotary cylindrical drum 114, and the printing sheet P and the rotary cylindrical drum 114 are brought into pressure contact with each other by the press roller 118, ink is passed through perforated portions of the stencil sheet M from inside the rotary cylindrical drum 114, thereby effecting printing on the printing sheet P.
The press roller 118 is rotatably supported by a support member 130. The support member 130 is fixed to a support shaft 131 which is disposed rotatably. Further, the support shaft 131 fixes a press arm 132. This press arm 132 is pulled in a counterclockwise direction in FIG. 5 about the support shaft 131 by a tension spring 133. As a result, the press arm 132 rotates the support shaft 131, which in turn causes the support member 130 fixed to the support shaft 131 to rotate counterclockwise in FIG. 5, thereby causing the press roller 118 to move to a pressure contacting position at which the press roller 118 is pressed against the outer peripheral surface of the rotary cylindrical drum 114.
At this pressure contacting position, the press roller 118 is pressed against the outer peripheral surface of the rotary cylindrical drum 114 by the tensile force of the tension spring 133, and thereby obtains printing pressure for causing the printing sheet P to be brought into pressure contact with the outer peripheral surface of the stencil sheet M wrapped around the rotary cylindrical drum 114.
Further, the press arm 132 has a cam follower 135 capable of being brought into sliding contact with a cam 136, which rotates about the same axis as that of the rotary cylindrical drum 114, by the tensile force of the tension spring 133. As the cam 136 rotates, the support shaft 131 is rotated clockwise in FIG. 5. As a result, the press arm 132 rotates the support shaft 131, which in turn causes the support member 130 fixed to the support shaft 131 to rotate clockwise in FIG. 5, thereby moving the press roller 118 to move to a retreated position at which the press roller 118 is spaced apart from the outer peripheral surface of the rotary cylindrical drum 114.
With the above-described stencil printing machine, however, the density of a printed image changes in cases such as where the viscosity of the ink has changed due to a temperature change, and the contact time of the printing sheet P with respect to the rotary cylindrical drum 114 (stencil sheet M) has changed due to a change in the printing speed. To cope with this situation, it is conceivable to adjust the density by making variable the printing pressure by the press roller 118, i.e., the tensile force of the tension spring 133.
However, the tension spring 133 is so designed as to be able to obtain a strong tensile force in order to stabilize the printing pressure by the press roller 118. For this reason, to make the tensile force of the tension spring 133 variable, a large-size driving mechanism (a motor, a gear, etc.) having a strong driving force overcoming the tensile force of the tension spring 133 becomes necessary, so that there are problems in that the cost becomes high, and that the size of the machine becomes large.
Accordingly, an object of the present invention is to provide a stencil printing machine which is capable of making the printing pressure variable after realizing the low cost and compact size, and which is capable of stabilizing the printed image performance in correspondence with the printing speed and the ambient temperature, thereby overcoming the above-described problems.
To achieve the above object, according to a first aspect of the present invention, there is provided a stencil printing machine which comprises a rotary cylindrical drum rotatable about its own axis, the rotary cylindrical drum including an inner surface to which ink is supplied, and an outer peripheral surface around which a stencil sheet subjected to a stencil making process is wrapped, a press roller movable into contact with and away from the rotary cylindrical drum, wherein a printing sheet is transported in synchronism with a rotation of the rotary cylindrical drum, and is brought into pressure contact with an outer peripheral surface of the stencil sheet by the press roller so as to effect stencil printing with respect to the printing sheet, a plurality of tension springs capable of applying printing pressure for pressing the press roller against the outer peripheral surface of the rotary cylindrical drum, and a tensile-force varying mechanism capable of extending or contracting one of the plurality of tension springs so as to render the printing pressure variable.
In the stencil printing machine according to the first aspect of the present invention, even if the viscosity of ink has changed due to a temperature change, and the contact time of the printing sheet with respect to the rotary cylindrical drum (stencil sheet) has changed due to a change in the printing speed, the printed image performance can be stabilized by adjusting the density of the image because the printing pressure is variable. In addition, a strong tensile force for stabilizing the printing pressure by the press roller can be dispersed by using the plurality of tension springs, and hence one of the dispersed tension springs may be extended or contracted. Accordingly, it is possible to reduce the driving force for rendering the printing pressure variable. Therefore, it is possible to realize a compact driving mechanism (e.g., tensile-force varying mechanism as described above) and a reduction of the cost.
Further, according to a second aspect of the present invention, the tensile-force varying mechanism of the first aspect may include a worm wheel capable of extending or contracting one of the plurality of tension springs in accordance with a rotation of the worm wheel, a worm capable of meshing with the worm wheel, a speed reducing gear capable of meshing with the worm, and a drive device capable of rotatively driving the worm through the speed reducing gear. Consequently, it is possible to hold the extended or contracted state of the tension spring without rotating the worm wheel by pulling back by the tensile force of the tension spring. In addition, since the speed reducing gear is interposed between the worm and the drive device in the tensile-force varying mechanism, the load on the drive device is alleviated, so that the adoption of a more compact drive device can be realized, thereby making it possible to attain a reduction of the cost.
Further, according to a third aspect of the present invention, the stencil printing machine of the second aspect may further comprise a sensor detecting piece, formed on the worm wheel, and a sensor capable of detecting a position of the sensor detecting piece to recognize a rotated position of the worm wheel.
Furthermore, according to a fourth aspect of the present invention, the stencil printing machine of the first aspect may further comprise a sensor capable of detecting a motion of the tensile-force varying mechanism so as to recognize a state of extension or contraction of one of the plurality of tension springs.